Previous research has supported the notion that uptake of oxygen (O2) by human erythrocytes (RBC's) in the lung is limited by the diffusion resistance of the RBC membrane to O2 and that intracellular O2 diffusion and the rate of reaction of O2 with hemoglobin (Hb) are not rate-limiting. Two bits of experimental evidence oppose this conclusion: a) the observation that thin layers of concentrated Hb solution take up O2 at the same rate as thin films of packed RBC's and b) the finding that steady-state O2 diffusion through a thin layer of concentrated Hb solution is as rapid as through a similar layer of packed RBC's. We propose to measure the oxygen permeability of the RBC membrane to determine if it limits oxygen uptake. RBC ghosts will be loaded with glucose oxidase, catalase, and glucose. This coupled enzyme system will consume O2 in the ghost and maintain the O2 tension near zero in the cell, thus allowing a rapid and sustained uptake of O2 that can be accurately measured. The idea that the passage of O2 through the RBC membrane is rate- limiting rests largely on the fact that RBC's in the rapid reaction apparatus take up O2 much more slowly than is calculated for RBC's with no membrane O2 diffusion resistance. The flow-type reaction apparatus depends on turbulent flow for mixing for the reactants. Since suspensions of RBC's are non-Newtonian, whether or not their flow is turbulent in the range of Reynold's numbers used is open to question. We propose to study the flow behavior of these RBC suspensions by means of the laser anemometer to determine the conditions under which there is actually turbulent flow in the rapid reaction apparatus. The discrepancy between measured and calculated rates of O2 uptake by RBC's can also be accounted for by RBC deformation in the shear gradients in the rapid reaction flow tube. We plan to use microsecond flash photography to study the distribution of red cell sizes and shapes in the flowing stream.